A plurality of transducer-channels has been the goal of audio engineers since the invention of the stereophonic art (a radiotelephony patent issued in 1924 to F. M. Doolittle) and its implementation in 1954 by the sound and motion picture industries. Since the first commercial recording of the stereophonic disc in 1958, the primary goal has been to resolve the Haas Effect by channelizing the phantom images residing between two transducers of a stereophonic sound field into point-source audio images. This phantom phenomenon has been referred to as the "stereophonic seat", and more recently as the "quadriphonic seat."
Numerous analog processing methods have failed in their attempts to channelize or enhance the directionality of phantom images. These methods, to mention a few, include: the algebraic sum derivation of a center channel (a method that failed to consider algebraic differences); a key signal detection method for emphasizing directionality (an impractical electromechanical method similar to SQ); an artificial high-passing and low-passing scheme (produced double sound images and listener confusion); an artificial time delay technique of delaying one channel for Haas effect derivation (several recording companies produced a limited quantity of recordings using this method); and a cosine derivation of multiple emphasized directions via a network of output power transformer taps (a 5-channel phase shifting method described in the March 1969 issue of "Audio").
In early 1970 algebraic matrix techniques made significant progress in multi-channel stereo reproduction. The first algebraic matrix system by Schieber was a "straight algebraic" method that provided a relatively poor separation of only 3 dB. Other algebraic matrix inventions, claiming to be improvements, did little or nothing more than change matrix factors to achieve slightly different directionality characteristics.
The "matrix wars" subsided as SQ (Columbia) and QS (Sansui) systems emerged as the dominant contenders. Both of these algebraic matrix systems, utilizing either "j" factors (90-degree phase shifts) or other matrix phase angle relationships (the Sansui "Variomatrix"), made substantial performance improvements over all previous algebraic matrix systems. In the SQ system, the 90-degree phase relationship is panpot-derived from a single-source audio signal and recorded on disc/tape as two identical "j" factor encoded (shifted) audio signals. These signals are then reproduced as a discrete audio image after "mirrored" 90-degree phase shift decoding. Both SQ and QS systems provide 4-channel phase shift decoding performance but neither solve the Haas effect for phantom images residing between any two transducers of a stereo field. In some respects the Sansui QS system is superior since its discriminator circuitry provides directional control for audio reproduction in a 360-degree field.
A new quadriphonic contender rekindled the "matrix wars" by employing a frequency multiplexing method (JVC Quadradisc). Since SQ or QS systems produced four transducer channels having limited channel separation, the JVC system was to provide superior separation or directionality. While this method proved to be feasible, it still exhibited erractic f.m. demodulation, limited phono-cartridge separation, and stylus tracking and record wear problems.
An improved SQ matrix system then evolved which utilized gain-riding logic that employed both side-to-side and front-to-back wave-matching logic. With this method, channel separation is equal to or better than the JVC multiplex system, but the SQ system inherently confuses directionality when all four channels require simultaneous reproduction. The JVC multiplex system maintains directionality for four channels of simultaneous reproduction.
Attempts have been made at affecting compatibility of the existing SQ system by utilizing amplitude modulated (a.m.) sideband multiplexing techniques; a method of compatibility for Columbia but a fifth system for the consumer. This method is significantly more susceptable to noise than the f.m. (JVC) multiplexing method.
Another approach to the systems previously described is a relative amplitude detection gate circuit that incorporates both an algebraic matrix and a logic circuit gate. This circuit attempts to recognize an amplitude ratio. From a signal processing standpoint, it fails to meet logic and analog circuit design guidelines or to provide processing functions such as: dynamic range compression, phase-angle decoding, peak-amplitude strobe synchronization, and flip-flop storage of the decoded amplitude ratio result during zero-crossover. This patented approach has yet to be put into practice.
In addition to previously mentioned quadriphonic methods, a discrete 4-track/Q8 tape system is available. The discrete 4-track tape system has been available since 1961 and is far superior to all previous methods for all aspects of functional performance; separation, signal-to-noise, and the like. However, this method is limited to the tape media while the bulk of the consumer market comprises the disc media.
To date; SQ, QS, JVC Quadradisc (having sufficiently resolved its previously stated problems), and discrete 4-channel/Q8 tape hardware systems and media attempt to coexist in the stereophonic-quadriphonic marketplace. This marketplace has a much curtailed consumer interest in both quadriphonic equipment and media since these four systems are not compatible, offer only 4-channel performance, and do nothing for the bulk of the recorded media . . . the 25-year consumer collection of stereophonic discs and tapes. And to compound this compatibility problem, the FCC is faced with deciding upon one of at least nine quadriphonic transmission methods before it sanctions 4-channel f.m.
The current state-of-the-art has been undergoing further improvement, such as: a shadow vector analysis unit for SQ; a paramatrix decoder by CBS; a Tate directional enhancement system alternative by CBS; a new system for cutting CD-4 masters (JVC Quadradisc); a CD-4 demodulator by Quadracast Systems, Inc.; and a JVC professional CD-4 demodulator. From the media standpoint, these improvements are resulting in a fragmentation of the 4-channel market by a number of companies attempting to promote their own matrix decoding/demultiplexing systems. None of the aforementioned systems or improvements can rival the performance made possible by this invention in terms of its flexibility, versatility, and performance/cost ratio.